Intermetallic Growth Research Articles

Kinetics of Isothermal Reactive Diffusion Between Solid Cu and Liquid Sn

The Cu/Sn system is one of the most fundamental and important metallic systems for solder joints in electric devices. To realize reliable solder joints, information on reactive diffusion at the solder joint is very important. In the present study, we experimentally investigated the kinetics of the reactive diffusion between solid Cu and liquid Sn using semi-infinite Cu/Sn diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of the diffusion couple was conducted in the temperature range of 533–603 K for various times up to 172.8 ks (48 h). Using annealing, an intermetallic layer composed of Cu6Sn5 with scallop morphology and Cu3Sn with rather uniform thickness is formed at the original Cu/Sn interface in the diffusion couple. The growth of the Cu6Sn5 scallop occurs much more quickly than that of the Cu3Sn layer and thus predominates in the overall growth of the intermetallic layer. This tendency becomes more remarkable at lower annealing temperatures. The total thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is close to unity at all the annealing temperatures. This means that volume diffusion controls the intermetallic growth and the morphology of the Cu6Sn5/Sn interface influences the rate-controlling process. Adopting a mean value of 0.99 for the exponent, we obtain a value of 26 kJ/mol for the activation enthalpy of the intermetallic growth.

A method of integrating CALPHAD data into phase-field models using an approximated minimiser applied to intermetallic layer growth in the Al-Mg system

This article describes an approach to combine single phase thermodynamic potentials into a multiphase composite potential suitable for integration in a multicomponent phase-field model. The composite potential avoids implicit interfacial energy contributions by starting from a grand potential formulation. The method is made explicit by expanding the minimiser resulting from the Legendre transform between grand and Helmholtz potentials about a known equilibrium state. The resulting composite function is explicit, reproduces the equilibrium states exactly, and is smooth such that it can be differentiated to provide the driving forces for mass transport and phase change in a thermodynamically self-consistent manner. The model is demonstrated by simulating a sequence of phase transformations for intermetallic growth in Al-Mg interdiffusion for advanced nuclear research reactor fuel.

Intermetallic Growth Retardation with the Addition of Graphene in the Sn-3.5Ag Lead-free Solder

In this study, the effect of 0.07 wt.% graphene nanosheets (GNSs) on the thermal, microstructure and intermetallic compounds (IMC) growth kinetics were investigated. Experimental results showed that addition of GNSs to the Sn-3.5Ag solder alloy slightly increased the melting temperature. Scanning electron microscope (SEM) micrograph revealed that with the addition of GNSs has transformed the morphologies of the interfacial IMC from scalloped to more planar-shaped after isothermal aging for 500 hours. Growth kinetics calculations show that the k value was reduced from 9.08 × 10-14 cm2/s to 5.73 × 10-14 cm2/s with the addition of graphene. Thereby, it indicates that the addition of GNSs suppressed the growth of the IMC layers hence retarded the IMC’s growth.

Kinetics of intermetallic phase growth and determination of diffusion coefficients in solid\u2013solid-state reaction between Cu and (Sn+1at.%Ni) pads

The influence of nickel addition to tin substrate on microstructure, kinetics parameters as well as diffusion coefficients was studied on Cu/Sn diffusion couples. The results revealed that the presence of nickel did not affect the growth of Cu3Sn phase in comparison with that of the binary Cu/Sn diffusion couple. However, it substantially influenced the morphology and chemical composition of the (Cu1−xNix)6Sn5 phase, which grew with dual morphology and different chemical composition. The layer of the phase was poorer in nickel, whereas detached grains were richer in nickel concentration. The calculated kinetic parameters showed that the growth of the Cu3Sn and Ni-poor (Cu1−xNix)6Sn5 phases was controlled by volume diffusion, but in the case of Ni-rich (Cu1−xNix)6Sn5 phase, the growth was controlled by a complex mechanism. The calculations of diffusion coefficients and activation energies of Cu3Sn and Ni-poor (Cu1−xNix)6Sn5 phases revealed that the presence of nickel in the tin substrate did not affect the rate of formation/growth of those phases in comparison with the binary Cu/Sn diffusion couples.

Microstructure and kinetics of intermetallic phase growth of three-layered A1050/AZ31/A1050 clads prepared by explosive welding combined with subsequent annealing

The effect of annealing has been investigated with respect to interface microstructure and evolution of intermetallic phases in three-layered explosively welded A1050/AZ31/A1050 specimens. Both interfaces in the state after welding were characterized by wavy shape morphology with intermediate phases, which formed segmented structures. Two different morphologies consisted of equiaxed and columnar grains were observed within the Mg2Al3 phase, while the Mg17Al12 phase was built of only columnar grains. Furthermore, a small amount of Mg23Al30 was detected by X-ray synchrotron diffraction. Annealing at 350°C strongly induced the Mg23Al30 phase development in the form of discontinuous layer between Mg2Al3 and Mg17Al12 phases after 10h of annealing. Kinetics calculations indicated that, Mg2Al3 phase growth at 300°C was controlled by different mechanisms according to the location: volume diffusion and chemical reaction at the upper interface and solely by volume diffusion at lower one. The growth of Mg17Al12 was governed only by volume diffusion. Furthermore, the same mechanisms were observed during annealing at 400°C, however this heat treatment significantly changed the microstructure i.e. the grain size and shape. It was also established that the nanohardness of both Mg2Al3 and Mg17Al12 was about 350HV.

Solid-state growth kinetics of intermetallic compounds in Cu pillar solder flip chip with ENEPIG surface finish under isothermal aging

Electroless Ni/electroless Pd/immersion Au (ENEPIG) constitutes a promising metallization to replace the conventional ENIG or Ni/Au for 3D IC integrated module and optoelectronic packaging. Different ENEPIG plating thicknesses with copper pillar solder joints are subjected to thermal aging at 150 °C to investigate the intermetallic compounds (IMCs) formation and growth. Due to low temperature solid-state bonding, both Pd and Au layers do not completely diffused into Sn matrix and participated in the interfacial reactions to form (Pd,Au)Sn4 IMCs phase. (Pd,Au)Sn4 IMCs based on PdSn4 phase with dissolved Au, exhibits high growth rate and substantial consumption of Sn from solder. Upon increasing the aging time, it is found that (Cu,Ni)6Sn5 IMCs and (Pd,Cu,Au)Sn4 phase form at interface and follows a diffusion control mechanism, which weakened the solder joint and caused brittle fracture in a die peel test. (Cu,Ni)6Sn5 IMCs growth rate increases with decreases Pd thickness. Therefore, the types of IMCs formation, growth rate, and reliability of Cu pillar joint strongly depend on bonding temperature, Au and Pd thicknesses, and solder volume. Based on this study, the authors recommend suitable ranges of Au and Pd layer thicknesses for reliable Cu pillar solder joints.

Microstructure evolution in Ni and Ni-superalloy based metallic-intermetallic laminate (MIL) composites

Pure nickel and Nickel Alloy-X based Metallic-Intermetallic Laminate (MIL) composites were fabricated and investigated for their microstructure evolution, growth kinetics and microhardness distribution across the intermetallic/metal interfaces. Multi-phase layer structure with parabolic growth kinetics is shown in the nickel-based MIL composites. A two-phase eutectic layer along with a uniform layer form in the Alloy-X based MIL composites, which shows a mixed growth kinetic mechanism. The microhardness distribution across the intermetallic/metal interface of the two MIL composites was investigate with respect to the effects of texture and microstructure of the intermetallic phases. A simple intermetallic growth model is developed and validated for predicting the phase formation in binary or ternary reactions for synthesis of these MIL composites.

The effect of temperature on the formation behavior of reaction layer in Sn‐3.0Ag‐0.5Cu solder joint with the addition of porous Copper interlayer

AbstractThis paper investigates the growth and formation of intermetallic compound when a porous copper interlayer is added into the soldering of 96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy and copper substrate. Porous copper interlayers with two different porosities, P15 (15 pores per inch, ppi) and P25 (25 ppi), were selected as interlayers to be placed in between 96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy/copper substrate. Three soldering temperatures of 267 °C, 287 °C and 307 °C with soldering time of 300 s were applied. It was found that during the soldering process, the intermetallic layer of copper compound with tin (6:5) with discontinuous scallop‐shaped morphology was formed at the interface of the 96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy and copper substrate, while a uniform and continuous intermetallic compound was observed at the interface of 96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy and porous copper. The activation energies of intermetallic compound growth at copper substrate/96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy interface for the samples without porous copper and with porous copper (P15 and P25) were calculated to be 52.90 kJ/mol, 54.53 kJ/mol and 51.67 kJ/mol, respectively. The activation energies of intermetallic compound growth at porous copper/96.5Sn‐3.0Ag‐0.5Cu (in wt%) solder alloy interface for P15 and P25 were 51.73 kJ/mol and 20.62 kJ/mol, respectively. Results of the tensile tests have shown that the highest joint strength for P15 was 54 MPa, whereas for P25 the highest joint strength was 52 MPa, both obtained at a soldering temperature of 307 °C.

Effect of heat treatment on morphology of Fe-rich intermetallics in Al–Cu alloys

The iron-rich intermetallics are considered to be stable in aluminium alloys and usually hard to dissolve into the matrix during solution heat treatment (SHT). Nevertheless, solid-state transformations between Fe-rich intermetallics may happen and will play a great role in breaking-up of the large Fe-rich intermetallics particle during hot deformation process. In this study, the solid-state phase transformation of the iron-rich intermetallics from AlmFe to β-Fe (Al7Cu2Fe) has been studied using SEM, XRD and deep-etched technique in a 2xxx Al–Cu cast alloy during SHT. The mechanism of solid-state transformation for the Fe-rich intermetallics was discussed, including the solid-state reaction, nucleation and growth of iron-rich intermetallics and Al phases.

38 - Enasidenib (AG-221), A Selective Oral Inhibitor of Mutant Isocitrate Dehydrogenase 2 (IDH2) Enzyme, In Patients with Myelodysplastic Syndromes (MDS)

Different types of Sn coatings are widely used in the electronics industry. However, they are susceptible to spontaneous whisker formation which can cause reliability issues. In this paper, whisker growth from submicron Sn thin films was studied at room temperature for 150 days. For this purpose, 99.99% pure Sn was vacuum evaporated onto 1.5 mm thick Cu and ceramic substrates, with an average thickness of 400 nm. It was found that the submicron thick Sn layers on a Cu substrate can generate intense whiskering directly after layer deposition. The microstructure, composition and grain orientation of the whiskers and the areas underneath were studied with a Scanning Ion Microscope (SIM) and a Transmission Electron Microscope (TEM). It was proven that the whisker growth was induced by Cu6Sn5 intermetallic layer growth between the Cu substrate and the Sn thin film which resulted in large stress within the film. We found the first evidence in a tin-copper layer system that the high stress due to the intermetallic formation itself can initiate the interfacial flow mechanism (without a direct mechanical load) between the Sn layer and the intermetallic layer. This phenomenon can explain the intense whisker growth from submicron Sn thin films.

Whisker growth from vacuum evaporated submicron Sn thin films

Different types of Sn coatings are widely used in the electronics industry. However, they are susceptible to spontaneous whisker formation which can cause reliability issues. In this paper, whisker growth from submicron Sn thin films was studied at room temperature for 150days. For this purpose, 99.99% pure Sn was vacuum evaporated onto 1.5mm thick Cu and ceramic substrates, with an average thickness of 400nm. It was found that the submicron thick Sn layers on a Cu substrate can generate intense whiskering directly after layer deposition. The microstructure, composition and grain orientation of the whiskers and the areas underneath were studied with a Scanning Ion Microscope (SIM) and a Transmission Electron Microscope (TEM). It was proven that the whisker growth was induced by Cu6Sn5 intermetallic layer growth between the Cu substrate and the Sn thin film which resulted in large stress within the film. We found the first evidence in a tin-copper layer system that the high stress due to the intermetallic formation itself can initiate the interfacial flow mechanism (without a direct mechanical load) between the Sn layer and the intermetallic layer. This phenomenon can explain the intense whisker growth from submicron Sn thin films.

Interface dynamics in one-dimensional nanoscale Cu/Sn couples

The isothermal metallurgical reaction in two-segmented Cu-Sn nanowires results in the formation of a Sn/Cu6Sn5/Cu3Sn/Cu sandwich structure. In-situ transmission electron microscopy is used to study how Cu6Sn5/Sn and Cu/Cu3Sn interfaces propagate and change shape during the intermetallic compound growth. The Cu6Sn5/Sn interface is observed to evolve from an inclined configuration to a vertical, edge-on configuration with the propagation of the Cu6Sn5 phase towards the Sn segment. The Cu/Cu3Sn interface also becomes less inclined as it propagates toward the Cu segment. This interface evolution is driven by the minimization of the interface energy associated with minimizing the interface area associated with the edge-on interface. The Kirkendall void growth induces the breakage of the Cu segment and results in the Cu3Sn → Cu6Sn5 transformation with the final Sn/Cu6Sn5/void/Cu sandwich structure.

Reliability enhancement of Sn-1.0Ag-0.5Cu nano-composite solders by adding multiple sizes of TiO2 nanoparticles

This paper demonstrates a novel low silver content TiO2 nanoparticle composite solder alloys, and discusses the TiO2 size effect on reliability of the solder and its solder joints with copper. 6 nm and 20 nm TiO2 nanoparticles were synthesized and added into Sn-1.0Ag-0.5Cu (SAC105) solder to prepare the nano-composite solders. The microstructure, coefficient of thermal expansion of the solders, and the shear strength, interfacial intermetallic compound growth of the solder joints under thermal cycling (1000 cycles) between −55 °C and 125 °C were investigated. The experimental results show that TiO2 nanoparticles can refine β-Sn grain size, decrease CTE of the solders, as well as suppress the interfacial intermetallic compound formation and growth. Also, after thermal cycling, the nano-composite solder joints have higher shear strength than the joints without TiO2 nanoparticles. Most importantly, smaller TiO2 nanoparticles (6 nm) shows larger influence on the respects of microstructure refinement, coefficient of thermal expansion reduction, interfacial IMC thickness decrease, and shear strength enhancement of the solder joints. The TiO2 nano-composite solder joints show higher electrical reliability than the original SAC105 solder joint according to the results of electric resistance change during thermal cycling.

Electromigration에 의한 Sn-0.7Cu 솔더 금속간화합물 성장 예측

Electromigration에 의한 Sn-0.7Cu 솔더 금속간화합물 성장 예측

Effect of cooling condition and Ag on the growth of intermetallic compounds in Sn-based solder joints

The intermetallic compound growth in Sn/Cu and Sn–3.5Ag/Cu solder joints undergoing cooling has been in-situ observed using synchrotron radiation X-ray imaging technique. The overall thickness of intermetallic compound attained during cooling condition is dependent on the rates of Cu precipitation or deposition from the bulk solder and Cu diffusion from grain boundary at interface. Although the net increase in IMC thickness contributed predominantly by deposition kinetics is greater for air cooling than in furnace cooling from the start temperature of $$300\,^\circ \hbox {C}$$ for the first 20 min, the former solidifies before 30 min and the latter stays in liquid state for 1 h due to slower cooling rate and attains a bigger IMC of size about 14.5 $$\upmu $$ m. In context of Sn–3.5Ag solders subjected to air cooling from 275 $$^\circ $$ C, the presence of Ag contributes to the increment in overall IMC thickness during the cooling period. For the improvement in solder joints reliability, faster cooling rate and limiting the Ag content can be employed as the materials design and processing parameters.

Effect of heat treatment on the interfacial microstructure and properties of Cu-Al joints

The purpose of this article is to study the effect of heat treatment on the interfacial microstructure and properties of the Cu-Al contact reaction brazed joint, and compare the results with a similar study performed on flash welding. The results show that the interfacial intermetallics in Cu-Al joint change from original two layers (Cu9Al4/CuAl2) to three layers (Cu9Al4/CuAl/CuAl2) both in contact reaction brazing and in flash welding. Some Kirkendall voids form during the growth of interfacial intermetallics. The voids in the contact reaction brazed joint form in the middle of interfacial intermetallic layer and lead a discontinuous crack after 500 h heat treatment. The voids in the flash welded joint form between copper and interfacial intermetallics and lead to a continuous crack after 500 h heat treatment. The growth rate of interfacial intermetallics conforms to the parabolic law for both joints. The relation between joint properties and the interfacial intermetallic thickness can be described by a linear function. The joint properties of contact reaction brazing are better than that of flash welding during the heat treatment. Therefore, contact reaction brazing is more preferred to maintain electrical stability and mechanical integrity of Cu-Al joint.

Effect of Solder Joint Thickness on Intermetallic Compound Growth Rate of Cu/Sn/Cu Solder Joints During Thermal Aging

The sandwich structure Cu/Sn/Cu solder joints with different thicknesses of the solder layers (δ) are fabricated using a reflow solder method. The microstructure and composition of the solder joints are observed and analyzed by scanning electron microscopy (SEM). Results show that the thickness of intermetallic compound (IMC) and Cu concentration in the solder layers increase with the decrease of δ after reflow. During thermal aging, the thickness of IMC does not increase according to the parabolic rule with the increase of aging time; the solder joint thickness affects markedly the growth rate of IMC layer. At the beginning of thermal aging, the growth rate of IMC in the thinner solder joints (δ ≤ 25 μm) is higher than that in the thicker ones (δ ≥ 30 μm). The growth rate of IMC (δ ≤ 25 μm) decreases in the thinner solder joints, while increases in the thicker solder joints (δ ≥ 40 μm) and is nearly invariable when the δ equals to 30 μm with aging time extending. The growth rate of IMC increases first and then decreases after reaching a peak value with the increase of δ in the later stage during aging. The main control element for IMC growth transfers from Cu to Sn with the reduction of size.

Intermetallic growth kinetics of 2N gold wire ball bonds on aluminium bond pads annealed at 175 °C

Gold ball bonding wires of 2N purity were bonded to Si devices with aluminium alloy bond pad metallisation. Specimens were subjected to isothermal annealing at 175 °C in a fan-circulated oven in an air environment for various times t. Samples of ball bonds from each specimen were cross-sectioned, and the total intermetallic thickness x at the centre of the ball bonds was measured. The kinetics of x versus t were modelled using a power law expression $$ x(t)={x}_0+{\theta}_1{t}^{\theta_2} $$ with growth parameter θ 1 and constrained parameter 0 < θ 2 ≤ 1. The parameter θ 2 is supposedly capable of differentiating between diffusion controlled intermetallic growth (grain boundary and/or volume diffusion) and interfacial reaction controlled growth with the presence/absence of grain growth solely based on its numerical value. The parameters θ 1 and θ 2 and the regression function were obtained by log-transformation of both the response variable x and the explanatory variable t and data fitting using simple linear regression (SLR), taking care to perform diagnostic checks to confirm that transformed data satisfied the requirements of SLR. Simulated intermetallic thickness data obtained by sampling from lognormal distributions was observed to closely approximate the experimental observations. Values of the exponent θ 2 were observed in the range 0.2–0.4, which was interpreted as indicative of grain boundary dominated interdiffusion with grain growth.

Reliability of Heavy Gage Aluminum Wire-bonds under High Temperature Aging

Abstract Thick aluminum (Al) wedge bonds have been widely used in analog microelectronic packages due their lower cost, ease of use and excellent thermal and electrical performance. Reliability of thick Al wedge bonds on bare copper (Cu) leadframes under high temperature aging conditions has not been studied at a fundamental level in the past. This study investigates the reliability performance of Al wedge bonds on bare Cu surfaces under isothermal aging conditions specified by the automotive grade product qualification requirements. Results obtained show that Al/Cu intermetallic formed at the interface is detrimental to the mechanical and electrical package reliability. Plating on the leadframe has been evaluated as a potential mitigation to prevent intermetallic growth and hence improve the overall wedge bond and package reliability under high temperature reliability testing conditions. Significant improvement in the wedge bond and package reliability was achieved by using nickel plating under the wedge bonds. The results presented in this study can be used to design a more robust package capable of meeting stringent automotive grade requirements.

Intermetallic compounds in solar cell interconnections: Microstructure and growth kinetics

The microstructure and intermetallic phase growth in solder joints of photovoltaic modules are investigated because of their significance for interconnection reliability. Interconnector ribbons with Sn60Pb40, Sn62Pb36Ag2, Sn43Bi57, Sn41Bi57Ag2 and Sn91Zn9 are soldered on the front busbars of industrial crystalline silicon solar cells. Cross sections are inspected using microscopy, SEM and EDX. The interconnections are isothermally aged, whereas the intermetallic layer thickness is determined successively. The microstructural changes in the bonds are characterized. Grain coarsening, volume increase of intermetallic compounds, Sn-penetration into the metallization and growth of grain boundaries between the phases are found. The composition of the intermetallic phases within Sn91Zn9-bonds is discussed. A diffusion model is used to simulate the intermetallic layer growth after 3000h at 85°C and thermal cycling from −40°C to 85°C for 600 cycles. A prognosis of the phase growth within the photovoltaic module after 25 years at the location Freiburg in Germany is made. It is found that the Ag3Sn-phase using Sn43Bi57 extends to 6.6µm after 3000h at 85°C as compared to 2.6µm for Sn60Pb40. After 25 years in Freiburg the Ag3Sn-layer within Sn43Bi57 joints is predicted to be 2.4µm whereas within Sn60Pb40-bonds only 2.2µm.